and a
meteor The central cubic
parsec around Sagittarius A* contains around 10 million
stars. Although most of them are old red
giant stars, the Galactic Center is also rich in
massive stars. More than 100
OB and
Wolf–Rayet stars have been identified there so far. They seem to have all been formed in a single
star formation event a few million years ago. The existence of these relatively young stars was a surprise to experts, who expected the
tidal forces from the central black hole to prevent their formation. This
paradox of youth is even stronger for stars that are on very tight orbits around Sagittarius A*, such as
S2 and
S0-102. The scenarios invoked to explain this formation involve either star formation in a massive
star cluster offset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gas
accretion disk around the central black-hole. Current evidence favors the latter theory, as formation through a large accretion disk is more likely to lead to the observed discrete edge of the young stellar cluster at roughly 0.5 parsec. Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec. This observation however does not allow definite conclusions to be drawn at this point. Star formation does not seem to be occurring currently at the Galactic Center, although the Circumnuclear Disk of molecular gas that orbits the Galactic Center at two parsecs seems a fairly favorable site for star formation. Work presented in 2002 by Antony Stark and Chris Martin mapping the gas density in a 400-
light-year region around the Galactic Center has revealed an accumulating ring with a mass several million times that of the
Sun and near the critical density for
star formation. They predict that in approximately 200 million years, there will be an episode of
starburst in the Galactic Center, with many stars forming rapidly and undergoing supernovae at a hundred times the current rate. This starburst may also be accompanied by the formation of galactic
relativistic jets, as matter falls into the central
black hole. It is thought that the Milky Way undergoes a starburst of this sort every 500 million years. In addition to the paradox of youth, there is a "conundrum of old age" associated with the distribution of the old stars at the Galactic Center. Theoretical models had predicted that the old stars—which far outnumber young stars—should have a steeply-rising density near the black hole, a so-called
Bahcall–Wolf cusp. Instead, it was discovered in 2009 that the density of the old stars peaks at a distance of roughly 0.5 parsec from Sgr A*, then falls inward: instead of a dense cluster, there is a "hole", or
core, around the black hole. Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory. For instance, although the black hole would eat stars near it, creating a region of low density, this region would be much smaller than a parsec. Because the observed stars are a fraction of the total number, it is theoretically possible that the overall stellar distribution is different from what is observed, although no plausible models of this sort have been proposed yet.
Stellar black holes The galactic center is suspected to have a large population of
stellar mass black holes. There are probably around 25,000 stellar mass
black holes in the central parsecs of the galactic center as a result of dynamical friction and migration. These black holes have a major effect on the
stellar population of the galactic center and the
S cluster. They limit the number of massive
O-type stars through stellar collisions. ==Gallery==